Biochemical reactions involve biocatalysts (i.e. microorganisms, plant and animal cells, enzymes) and result in the transformation and production of biological and chemical substances. Vessels and apparatus (bioreactors) are required so that living organisms or enzymes can exhibit their activity (specific biochemical and microbial reactions) under defined conditions. In immobilized biocatalyst reactors, the biocatalysts may be immobilized in or on a carrier, immobilized by linkage among one another to form larger particles or confined within membrane barriers. Most of the reactors can be run in a batch, fed-batch or continuous mode.
Hitherto known equipments for using immobilized biocatalysts are the conventional reactors such as Continuous Stirred Tank Reactors (CSTR) and Packed Bed Reactors (PBR) as described in standard text books such as Ullmann's Encyclopedia Of Industrial Chemistry: Fifth edition, T. Campbell, R. Pfefferkom and J. F. Rounsaville Eds, VCH Publishers 1985, Vol A4, pp 141–170; Ullmann's Encyclopedia Of Industrial Chemistry: Fifth ed., B. Elvers, S. Hawkins and G. Schulz Eds, VCH Publishers, 1992, Vol B4, pp 381–433; J. B. Butt “Reaction Kinetics And Reactor Design” Prentice-Hall, Inc., 1980, pp 185–241.
The continuous stirred tank reactors consist of a tank containing a stirrer and, usually, fixed baffles to improve mixing. In a CSTR the immobilized enzyme is stirred with the substrate solution at fixed rpm and temperature. The reaction is monitored by appropriate technique and when the reaction is complete, the enzyme is separated from the reaction mixture by filtration and recycled. The CSTRs used for enzyme-catalyzed reactions assume a variety of configurations depending on the method employed to provide the necessary enzyme activity.
One of the popular ways of immobilization of an enzyme is to use an ultrafiltration membrane with pores sufficiently small to prevent the escape of the relatively large enzyme molecules in the solution However, the technique is useful only in cases of enzymes that have long term stability in solutions and are relatively inexpensive and hence, expendable.
Another technique is retention of immobilized enzymes in solution using a screen. A screen in the effluent line suffices if the enzyme is immobilized on insoluble particles, which are suspended in the reaction mixture as slurry. However, in such a system, the immobilized enzyme particles undergo attrition resulting in loss of enzyme as fines.
Yet another way of using immobilized enzyme in a stirred tank reactor is to employ pellets of immobilized enzyme held in a perforated container attached to an impeller. This configuration, which has also been widely used for the study of gas-phase reactions on supported metal catalysts, is intended to minimize mass-transfer resistance between the liquid phase and the immobilized-enzyme pellets. However, the size of the particle becomes very important in such cases and can lead to severe external mass transfer limitations.
Packed bed reactors are also used for biocatalytic processes. These reactors contain a settled bed of immobilized enzyme particles. The reaction mixture enters continuously from one end and the product moves out from the other end of the reactor. These reactors are like columns, and the degree of reaction, for a fixed flow rate, is proportional to the length of reactor column. A turbulent flow of reaction mixture through the column is preferred as it improves mixing. These reactors are preferred only in cases of processes involving product inhibition, substrate activation and reaction reversibility. However, colloids or precipitates formed during the reaction may clog up packed bed reactors. Also, temperature and pH are not easily regulated. Due to compact packing, excessive pressure drops are encountered which form the major bottleneck for the packed bed reactors. Channelling is also encountered which leads to improper contact between the biocatalyst and the reactants.
In the fluidtzed bed bioreactors the immobilized enzyme particles are fluidized, i.e., the particles become suspended in substrate stream, by the flow of the substrate stream. The immobilized enzyme particles are usually quite small, e.g., 20–40 μm in diameter, if their density is sufficiently high, otherwise larger particles have to be used to prevent them from being flown out of the reactor These reactors have kinetic properties between continuous flow stirred tank reactors and packed bed reactors. Fluidization of the bed requires a large power input, and such reactors are difficult to scale up. These reactors also need very high flow rates causing attrition of the biocatalyst and loss of the enzyme activity.
A membrane reactor uses a membrane, for e.g., a dialysis membrane, to contain the enzyme in a chamber into which the substrate moves and the product moves out. Each reactor contains hundreds of such fibres into which the enzyme is retained. The substrate is kept in the main chamber of the reactor. The substrate flow is adjusted to achieve the desired level of conversion. These reactors are easy to establish, permit the use of more than one enzyme to catalyze a chain of reactions, allow easy replacement of enzymes and are useful in producing small-scale (g to Kg) quantities. The chief limitations of these systems are: Regular replacement of membranes adds to cost and the need for substrate diffusion through the membrane often limits applications.